In the formation of substantially vertical concrete structures, it is well known in the art to use forms, usually made of wood, for molding concrete. Traditionally, the wooden forms are constructed with interior dimensions and shapes exactly equal to the concrete wall to be poured and formed within. That is, once the poured concrete fills the form, it is left to dry, and with the form removed, the concrete will remain in the desired form. Such wooden forms must be constructed with substantially durable and heavy gauge plywood, so that the intense pressure (due to the heavy weight of concrete) pent-up within the form is contained. Such wooden forms are routinely constructed at great expense, and require a great deal of skill to set up and disassemble once the concrete is dry. Furthermore, upon disassembling the forms, it is common that parts of the interior hardened concrete can become damaged if the form is not carefully and properly removed.
Many other shortcomings exist with the use of traditional forms that are assembled like boxes. "Box" forms must be constructed with all sides in place, thus leading to relatively high material costs for constructing forms generally. Also, because overall construction costs are generally driven by time constraints, the additional time required for assembling, placing and disassembling box forms results in substantial costs and diminished productivity.
When relatively high and narrow concrete structures are formed with correspondingly high and narrow box forms (for example, a ten foot (10') high by four inch (4") wide wall), other problems can result from the lack of access to the area in which the concrete is being poured. For example, if in the case of pouring a 10' high by 4" wide wall, a cement mixer fails, pouring must be halted. This can happen even though the wall formation may only be barely underway. In this event, although iron support rods or rebar will provide support between the wall segments which were poured at separate times, it is desirable to install a water barrier between the two wall segments. For this purpose, water barriers or "water stops" are often partially placed into the upper most surface of the recently poured, wet concrete. Then, when pouring is recommenced, the protruding portion of the water stop protrudes into the next segment of the concrete wall that is being poured. Such water barriers are often made of rubber or some type of polymer, and are generally desirable whenever an unexpected "cold joint" occurs. Unexpected cold joints are common in the practice of pouring concrete walls, but the insertion of water barriers is often not possible. For example, in the case of deep and narrow box forms, it is often impossible to have access to the uppermost surface of the freshly poured concrete, as it could be several feet down into the form. Thus, box forms have many shortcomings.
One way to avoid the use of box forms is through the use of "slip-type" forms, as described by M. K. Hurd, Formwork For Concrete, published by the American Concrete Institute in 1973. In the article, a special technique for concrete construction is set forth, using a slip form. The major advantages of such slip forms are speed and cost. With respect to slip forms used to create substantially vertical walls, two opposing forms traditionally travel along the vertical length of rails to form a vertical concrete wall between the two forms. The two opposing forms travel continuously upward as the concrete is inserted between them. This type of slip form, that is, the two-sided slip form, can only be used where few projectiles traverse the sliding direction, if not, one of the two sliding members could become impacted against the protruding projectiles.
Another problem with traditional two-sided slip forms entails the cost of constructing two separate form-slider mechanisms. That is, two redundant forms each associated with sets of rails must be constructed, one form and set of rails for each side of a wall to be formed. Also, where an embankment or wall is already existent (that is, a so-called "base wall" exists), it is most desirable to form concrete walls against such base walls. That is, when a two-sided form is used to construct a vertical wall, the newly formed wall must necessarily be bolted or otherwise fastened to the previously existent base wall. Such bolts require that holes be drilled through both adjacent walls, so they can be fastened together. The bolt holes can lead to water or thermal leakage. Also, over periods of time, such bolts can become corroded, thus affecting the structural integrity of the overall wall. By forming a concrete wall against a base wall, the two walls, in effect, would become fused together, since the wall being formed drys as it is pressed up against the base wall, leading to a sealed (air and water tight) unit when the poured wall dries. If a two-sided slip form is used, such an arrangement is impossible, since one side of the two-sided form will always be present between the existent walls or embankments (base walls) and the newly formed wall.
Another problem with two-sided slip forms is that the two forms must be positioned opposed to each other, and able to withstand intense pressure, resulting in higher construction costs.
Finally, with respect to positioning concrete against existent or base walls, care must be taken not to cause loose dirt or gravel to slide into the concrete mix. That is, if the ground above the drop-off of an embankment is disturbed, loose dirt or gravel may enter the area where the concrete is to be formed, causing impurities to enter the concrete wall. For example, if a cement mixer is positioned immediately above a gravel embankment, there could be a tendency to cause loose gravel to fall into the area below where the concrete is to be poured and formed. Although a two-sided form can help prevent this occurrence (one side of that type of form lies between the forming area and the embankment), constructing a one-sided forming system would be especially susceptible to gravel contamination, where concrete is placed into the forming area directly by a concrete truck or workers by shovel who are positioned at the rim of the embankment.
Other problems exist in construction with traditional concrete wall forming techniques. Slip forms, for example, are often over sized to accommodate a maximum amount of concrete at any given instant in time. This often leads to protracted setting and drying times. Also, when concrete is manually shoveled into a form, it is often first dropped (often from a shovel and wheel-barrow) onto a sheet of plywood adjacent to the opening in a wooden form, before ultimately being shoveled into the concrete forming area within the form. Problems have been experienced in this regard, however, because the cement and water often separate from the gravel (or rocks), thus sacrificing concrete strength and quality. This is often the case, even where traditional slip-forming is used, because slow concrete delivery systems (e.g., by hand) cannot keep up with the forming process.
Overpouring into the slip forms, also problematic, results in not being able to move the slip form upward in time before the freshly poured concrete begins to adhere to the form. Generally, the more concrete that is poured against the forms, the more difficult it is to move the forms, due to frictional forces.